8 



Description of an Automatic 



barometer tube, and passes down in the mercury on tbe 

 side of the float b. This wire is also connected with one 

 pole of the battery. 



The principle employed for giving motion to the screw 

 S, which follows the fluctuations of the mercurial column, 

 has been taken from the stop work long used on clocks. 

 The barrel of a clock on which the cord is wound usually 

 has a one-tooth wheel on its axis ; and at every revolution 

 of the barrel, a cog wheel is made to advance one tooth. 

 This cog wheel is, of course, always detached from the 



Fig. 2. 



